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The question is: what directories of file system I should consider to be mounted as ro (as maximum as possible), and what as rw to use Ubuntu as normal? I want to flash as maximum directories as I can to unerasable ultra-fast storage. Upgrading and other stuff is not normal :)

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do you have ram on your system ? – Antoine Rodriguez Oct 13 '13 at 0:23
Yes, I have ram (so, I can create a little disk using tmpfs or zram). I just want to make Ubuntu boot from immutable storage. Also, i don't want to use aufs, unionfs or smth. Well, FHS permits some directories to be mounted as readonly, but I cannot find what are they. – Alexander Shishenko Oct 13 '13 at 0:46

Instead of doing it yourself, why not using the live cd of ubuntu ?

Here is a tutorial on how to boot the live cd of ubuntu from the hard drive (in your case your flash drive).

And here is a tutorial on how you can customise a live cd :

Best regards.

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No, I have a really huge piece of software on an immutable storage disk, it can't be fit on a single CD or DVD or USB. The point is, that this storage must be shared between multiple Ubuntu installations. So, I want to install it once, burn to disk, and use multiple machines simultaneously using this storage. – Alexander Shishenko Oct 13 '13 at 1:04
the physical media has restriction of size but not the iso file :) You can have a 10Gb Iso file if you want. you'll be using only the iso file of the live cd that you'll customise with your software/data. – Antoine Rodriguez Oct 13 '13 at 1:38

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